Method for calibrating an ultrasonic measuring device intended for analysing a liquid metal

Abstract

Disclosed is a method for calibrating an ultrasonic measuring device (1), the device being intended to characterise a liquid metal, the probe comprising: - a measurement emitter (11) configured to emit an incident ultrasonic wave into the liquid metal; - a receiver (21) configured to detect an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal, following emission of the incident ultrasonic wave; and - a processing unit, configured to determine the amplitude of the detected ultrasonic wave, the method being characterized in that it comprises a phase of calibrating the probe by applying, upstream of the probe, a standing acoustic wave propagating in the liquid metal so as to retain inclusions by acoustophoresis.

Description

[0001] Description

[0002] Title: Method for calibrating an ultrasonic measuring device intended to analyze a liquid metal.

[0003] Technical field

[0004] The technical field of the invention is the characterization, by ultrasound, of a casting of liquid metal, for example a casting of an aluminum alloy.

[0005] Prior art

[0006] In the field of metal casting, it is necessary to control the quality of the liquid metal in order to detect inclusions. This is particularly the case for aluminum. Some industries, for example the housing industry, have high requirements regarding the quality of molten aluminum alloys. The quality of the molten metal is determined by the amount of inclusions present in the metal. These can be traces of gases, oxides, nitrides, carbides, fluorides or borides. Common inclusions found in molten aluminum are, for example, AI2O3, MgALC, AI4C3, MgO, CaO, CaF2, TiE, TiVE, SiC>2.

[0007] It is possible to monitor the quality of a casting online using ultrasonic measurement techniques. In this type of measurement, an ultrasonic acoustic wave propagates through the liquid metal. The acoustic wave propagates through the liquid metal. In the presence of inclusions, a portion of the emitted acoustic wave is reflected and can be detected. The amplitude of the reflected or diffracted wave can be used to determine the size of the inclusions in the metal. The ultrasonic technique allows for characterization of a portion of the liquid metal, typically 2%.

[0008] Probes for carrying out measurements, by ultrasound, on castings, have been described in patents EP3204764B1, EP3204763B1.

[0009] Document EP1194772B1 describes a method for calibrating a probe, in which reflectors are placed at the end of rods, the latter being arranged in liquid metal. The reflectors can be of different sizes. It is thus possible to establish a correlation between the size of each reflector and the amplitude of the detected acoustic wave. Such a method needs to be applied for each probe. In addition, it assumes the arrangement of calibrated reflectors in a liquid metal, which can be complex.

[0010] The document US20190145939 described relates to a method and a device for quantitatively determining the number and size of particulate components contained in a medium flowing along a flow channel; ultrasonic waves are coupled to the flowing medium, which are at least partially reflected by the particulate components and the reflected ultrasonic wave parts are detected in ultrasonic time signals, on which the quantitative determination is based.

[0011] US4770699 describes a probe for ultrasonic testing or processing of molten metal which comprises: an ultrasonic transmitting rod, one end of which is a "downstream" end adapted to contact the molten metal being tested or processed; a piezoelectric crystal adapted to provide ultrasound to the rod or to receive ultrasound from the rod; and rod cooling means located proximate the downstream end of the rod.

[0012] Document US5604301 describes a method of separating a liquid and in particular a liquid metal from heterogeneous constituents therein comprising the steps of providing a passage for a stream of liquid containing heterogeneous constituents and directing ultrasonic waves into the passage.

[0013] Another possibility for estimating the size of inclusions is to use a correlation curve between the measurements resulting from an ultrasonic probe and measurements resulting from a method considered as a reference method. The reference method may for example be an analysis method of the "LIMCA" (Liquid Metal Cleanliness Analysis) type, based on an analysis of the electrical resistance of a sample of liquid metal. It may also be the method described in EP1194772B1, using calibrated reflectors. It is thus possible to establish a correlation function between an inclusion size, measured by the reference method, and the amplitude of the ultrasonic signal.

[0014] A difficulty with using ultrasonic probes is that their response can vary from one probe to another. However, it is difficult to envisage calibrating each probe with a reference method, such as LIMCA or as described in EP1194772B1. Such calibration requires time.

[0015] The variability of the response between two probes, with identical structures, is due to differences that may affect the electronic circuits or the materials used. Faced with identical inclusions, two different probes, with identical design, can generate detection signals of different amplitudes, due to the variability of the instrument response.

[0016] It is preferable, even necessary, to take such variability into account, for example if one wishes to apply the same correlation curve to measurements resulting from different ultrasonic probes.

[0017] The inventors propose a method for taking into account the variability of responses of two different probes. The objective is that two probes, of identical design, generate identical signals, or signals that can be considered as such, in the presence of identical inclusions.

[0018] Statement of the invention

[0019] A first object of the invention is a method of calibrating an ultrasonic measuring device, the device being intended to characterize a liquid metal, the device comprising:

[0020] - a measuring transmitter configured to emit an incident ultrasonic wave into the liquid metal;

[0021] - a receiver configured to detect an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal, following the emission of the incident ultrasonic wave;

[0022] - a processing unit, configured to determine an amplitude of the detected ultrasonic wave; the method being characterized in that it comprises the following steps: a) arranging a sonotrode in a liquid metal, and arranging the device, such that the measuring transmitter and the receiver are arranged in the liquid metal or facing the liquid metal; b) activating the sonotrode, during an activation period, so as to generate a standing acoustic wave propagating through the liquid metal; c) powering the measuring transmitter, such that the measuring transmitter emits an incident ultrasonic wave propagating in the liquid metal at different calibration times, the calibration times forming a calibration period;d) at each calibration instant, detection, by the receiver, of an ultrasonic wave reflected or diffracted by the liquid metal under the effect of the incident wave, and determination of the amplitude of the ultrasonic wave detected by the receiver; e) calculation of a normalization amplitude as a function of the amplitude measured at each calibration instant. f) calculation of a normalization function, configured to normalize each amplitude of an ultrasonic wave detected by the receiver by the normalization amplitude.;

[0023] The standing acoustic wave can be an ultrasonic wave.

[0024] The device can be placed at a distance from the sonotrode of less than 2 meters.

[0025] The frequency of the standing acoustic wave may be different from the frequency of the incident wave.

[0026] The calibration period may be later than the activation period.

[0027] According to one possibility, the sonotrode forms the measuring transmitter. The process is then such that: - during step b), the measuring transmitter emits the standing acoustic wave;

[0028] - during step c), the measuring transmitter and the receiver of the device are activated, so as to determine the amplitude of the ultrasonic wave detected by the receiver.

[0029] According to one possibility, the calibration period extends to a duration less than ls (second).

[0030] According to one possibility, the frequency of the standing acoustic wave, generated during step b), is higher than the frequency of the incident ultrasonic wave emitted during step c).

[0031] According to one possibility:

[0032] - the frequency of the standing acoustic wave, generated during step b), is different from the frequency of the incident ultrasonic wave emitted during step c).

[0033] - the calibration period includes all or part of the activation period, all or part of steps b) and c) being implemented simultaneously.

[0034] A second object of the invention is a method for characterizing a liquid metal, the method using an ultrasonic measuring device, comprising:

[0035] - a measuring transmitter configured to emit an incident ultrasonic wave into the liquid metal;

[0036] - a receiver configured to detect an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal, following the emission of the incident ultrasonic wave;

[0037] - a processing unit, configured to determine an amplitude of the detected ultrasonic wave; the method comprising the following steps:

[0038] - (i) arrangement of the device, such that the measuring transmitter and the receiver are arranged in the liquid metal or facing the liquid metal;

[0039] - (ii) powering the measuring transmitter, so that the measuring transmitter emits an incident ultrasonic wave propagating in the liquid metal;

[0040] - (iii) detection, by the receiver, of an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal under the effect of the incident wave;

[0041] - (iv) at a measurement instant, measurement, by the receiver, of an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal under the effect of the incident ultrasonic wave, and determination of the amplitude of the ultrasonic wave detected by the receiver at each measurement instant; the method being characterized in that it comprises (v) application of a normalization function to the amplitude determined at the measurement instant, the normalization function being defined by a method according to the first subject of the invention.

[0042] According to one possibility:

[0043] - the liquid metal flows along a chute;

[0044] - the transmitter and receiver are positioned facing the chute.

[0045] According to one possibility:

[0046] - the measuring transmitter is coupled to a first waveguide, immersed in the liquid metal;

[0047] - the receiver is coupled to a second waveguide, immersed in the liquid metal.

[0048] A third object of the invention is an ultrasonic measuring device, comprising:

[0049] - a measuring transmitter configured to emit an incident ultrasonic wave into the liquid metal;

[0050] - a receiver configured to detect an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal, following the emission of the incident ultrasonic wave;

[0051] - a processing unit, configured to implement steps e) and f) of a method according to the invention.

[0052] The invention will be better understood by reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below.

[0053] Figures

[0054] Figure 1A represents a first embodiment of a probe allowing implementation of the invention.

[0055] Figure 1B represents another embodiment of a probe allowing an implementation of the invention.

[0056] Figure 2 shows a diagram of acoustic waves detected at a measurement time.

[0057] Figure 3 schematically represents a standing acoustic wave extending through a chute.

[0058] Figure 4 represents a correlation curve between detected wave amplitudes and an inclusion dimension.

[0059] Figure 5 shows the main steps of a method according to the invention.

[0060] Figure 6A shows a diagram of a sonotrode immersed in a chute.

[0061] Figure 6B shows a temporal evolution of the concentration of inclusions of different sizes in a casting, with and without application of a standing acoustic wave, during a first test. Figure 6C shows a temporal evolution of the concentration of inclusions larger than 20 pm in a casting, with and without application of a standing acoustic wave, during a second test.

[0062] Figure 6D and Figure 6E show histograms of the inclusion size distribution, with and without application of a standing acoustic wave.

[0063] Figure 6F shows a temporal evolution of the concentration of inclusions larger than 20 pm in a casting, with and without application of a standing acoustic wave, during a third test.

[0064] Figure 7A and Figure 7B are images resulting from PODFA (Porous Disk Filtration Analysis) analyses of samples taken from a flow respectively without and with application of a standing acoustic wave.

[0065] Figure 8A shows the evolution of a detection signal resulting from the probe during an experimental test.

[0066] Figure 8B shows an enlargement of the boxed area in Figure 8A.

[0067] Presentation of specific embodiments

[0068] A first example of an ultrasonic probe is shown, in connection with Figure 1A, enabling implementation of the invention. The probe comprises a measurement transmitter 11 and a receiver 21. In this example, the measurement transmitter 11 and the receiver 21 are formed by the same acoustic transducer. The acoustic transducer is, for example, a piezoelectric transducer, known to those skilled in the art.

[0069] The transducer is, for example, a pellet of piezoelectric material emitting an acoustic wave at a frequency of 15 MHz. Generally speaking, the frequency of the emitted acoustic wave is preferably between 1 MHz and 20 MHz.

[0070] The probe is intended for detecting inclusions 4 in a liquid medium 2 to be analyzed, and more precisely in a liquid metal, the metal being for example an aluminum alloy. In Figure 1A, the dotted horizontal line represents an interface between the liquid metal 2 and an ambient medium 3, the latter being for example air.

[0071] The probe 1 comprises a waveguide 13, intended to be immersed in the liquid metal 2. The waveguide is configured to facilitate propagation of an acoustic wave between the transducer, acting as a transmitter-receiver, and the liquid metal 2. The waveguide 13 is for example formed from a material of the silicon nitride or silicon oxynitride (SiAION) type, or more generally a material, of the refractory metal or ceramic type, considered to be inert to the liquid metal 2. The use of such a waveguide has been described in

[0072] EP3204763B1 or in EP1194772B1.

[0073] In Figure 1A, an ultrasonic field 5 is shown diagrammatically, which corresponds to a part of the liquid metal inspected by the probe 1. When an inclusion 4 passes through the ultrasonic field, a part of the acoustic wave emitted by the measuring transmitter 11 is reflected or diffracted towards the receiver 21. The characteristics of the reflected wave, in particular its amplitude, allow the detection of an inclusion and its characterization, for example an estimation of its size.

[0074] Preferably, the probe may comprise a power transmitter 12, configured to emit an acoustic wave of power, i.e. greater than 10W, and preferably greater than 50W or 100W, for example 120W, as described in EP3204764. Indeed, the application of a high-power acoustic wave makes it possible to obtain the wetting of the first waveguide 13 by the liquid metal 2. The power transmitter 12 is activated at regular intervals, so as to allow good wetting of the waveguide 13 by the liquid metal 2 to be maintained.

[0075] Figure 1B represents a preferred configuration, in which the probe comprises a measurement transmitter 11 and a receiver 21. The measurement transmitter 11 is connected to a first waveguide 13. The receiver 21 is connected to a second waveguide 23, preferably identical to the first waveguide 13 (same material, same dimensions), but distinct from the latter. The length of each waveguide is for example between 10 cm and 50 cm. The first and second waveguides are preferably rectilinear, and extend respectively around a first central axis A1 and a second central axis A2. The central axes A1 and A2 are intersecting, and inclined relative to each other at an angle a. The angle is for example between 20° and 40° and preferably between 25° and 35°. The angle a is for example equal to 28°. Such a configuration has been described in EP3204764B1.

[0076] In this embodiment, the probe may comprise a first power emitter 12, and a second power emitter 22, as described in connection with the first embodiment. The first and second power emitters are activated periodically, so as to enhance the wetting of the first waveguide 13 and the second waveguide 23, respectively, by the liquid metal 2.

[0077] The probe comprises a control unit 31, configured to allow control of the emission of an acoustic wave by the measurement transmitter 11, and to allow control of the emission of a power acoustic wave by the power transmitter 12 when the probe comprises such a transmitter. With regard to the acoustic waves emitted by the measurement transmitter 11, they are generally emitted in series. In practice, each series of acoustic waves is emitted by the measurement transmitter 11 at a regular frequency, for example 0.1 Hz. A series comprises at least one acoustic wave, and preferably several successive acoustic waves. Within the same series, the emission frequency of the acoustic waves can be between 100 Hz and 300 Hz, which corresponds to the excitation frequency of the piezoelectric transducer of the measurement transmitter.

[0078] In Figure 2, three waves detected by the receiver 21 are shown diagrammatically, following the emission of a series of three waves by the measuring transmitter 11. The bracket indicates that the three waves belong to the same series. In Figure 2, the ordinate axis corresponds to the signal S(t) detected by the receiver 21 while the abscissa axis corresponds to time t.

[0079] The probe comprises a processing unit 32, configured to calculate an amplitude of the waves detected by the receiver 21 in response to a series of acoustic waves emitted by the measuring transmitter 11. The calculated amplitude can be an average of the maximum amplitudes of each detected acoustic wave, or the maximum amplitude of all the acoustic waves detected following the emission of a series of waves. Subsequently, the calculated amplitude is noted (t m ). t m designates a measurement instant, which corresponds to the detection of a series of acoustic waves by the receiver 21.

[0080] When the amplitude A t m ~) crosses a certain threshold, we are in the presence of an inclusion 4 in the liquid metal. The amplitude (t m ) can make it possible to estimate an inclusion size, from an experimentally established correlation curve, using a reference method, for example the LIMCA method or the method based on the use of calibrated reflectors, these methods being described in connection with the prior art.

[0081] The same correlation curve can be used on different ultrasound probes of identical design, provided that the amplitudes resulting from said ultrasound probes are comparable. However, as previously indicated, it is necessary to take into account the variability of the probes, even if they are similar to each other: same dimensions and same materials used.

[0082] The processing unit 32 comprises an amplifier, intended to amplify the signal resulting from the receiver 21, and whose amplification gain Go is generally set in the factory, following the manufacture of the probe. The gain is generally not modified during the procedure. It may be subject to adjustment depending on the conditions of use of the probe. The probe 1 is intended to be immersed in a liquid metal 2 flowing from a furnace. For example, the liquid metal flows along a chute. The waveguide(s) of the probe are immersed in the chute.

[0083] For reasons of cost and time, it is not realistic to perform, for each probe, a correlation curve with a reference method. However, it is necessary to take into account the variability of the response of each probe. The inventor proposes a method making it possible to obtain a comparable response for each probe, so as to be able to use a correlation curve as described below, in connection with Figure 4.

[0084] An important aspect of the invention is to be able to have, within a casting, a liquid metal in which the concentration of inclusions is controlled, in order to carry out a so-called calibration measurement using an ultrasonic probe. The underlying idea is that for the same concentration of inclusions, the measurement amplitude must be the same, regardless of the probe used.

[0085] However, the concentration of inclusions in a casting varies in time and space. In order to obtain a controlled concentration of inclusions, the casting is subjected to a standing acoustic wave, at at least one point of the casting. The inclusions are then placed in an acoustic pressure field, and undergo a force, called acoustic force, which can cause a displacement of said inclusions. The inclusions can concentrate in the pressure nodes, i.e. the areas in which the amplitude of the pressure wave is minimal, or in the pressure antinodes, i.e. the areas of the sample in which the amplitude of the pressure wave is maximal. It has also been observed that the inclusions concentrate at the interfaces, whether these are the walls delimiting the chute or the free surface of the liquid metal.Thus, the use of an acoustic wave allows the inclusions to be moved, without physical contact, in order to impoverish the observation field of the probe, that is to say to reduce the quantity of inclusions present in the observation field.

[0086] The invention is based on the generation of a standing acoustic wave at certain points of the casting, upstream and / or downstream, or at the level of a probe. According to the principles of acoustophoresis, the standing acoustic wave makes it possible to move and / or retain inclusions flowing in the casting, outside the field of observation of the probe 1. Thus, the field of observation of the probe can be considered as devoid of inclusions.

[0087] According to one possibility, the sonotrode is arranged at a distance from the probe (or more precisely from the field of observation of the probe), upstream or downstream of the probe, the upstream and downstream being defined relative to the flow of the liquid metal. The inclusions are then trapped, in particular at the free interface (liquid metal / air interface) or at the walls of the chute. The distance between the probe and the sonotrode is then preferably greater than 10 cm, and for example between 30 cm and 2 m. This distance is adjusted by the person skilled in the art, so that it is sufficient so that the standing acoustic wave is not detected by the probe, while being sufficiently close, so that the filtration effect exerted by the standing wave is used. In Figure 3, a standing acoustic wave W extending perpendicularly to a flow direction of a stream of liquid metal 2 flowing in a chute G is shown.The inclusions move towards the free interface 2' (liquid metal / air interface) or towards the wall 6 of the chute.

[0088] Alternatively, the sonotrode is formed by the emitter 11 of the probe. In this case, the emission of the standing acoustic wave by the emitter induces a displacement of the inclusions outside the field of observation, at the level of the free interface or the walls of the chute.

[0089] Regardless of the embodiment, the standing acoustic wave is emitted at a high frequency, typically greater than 10 KHz, or 20 KHz if noise pollution is to be limited. This is considered to be more effective with respect to small inclusions. In addition, the application of a standing acoustic wave in the ultrasonic domain makes it possible to limit noise pollution.

[0090] The standing acoustic wave W generated upstream of probe 1 acts as a filter, allowing inclusions to be moved and / or maintained outside the field of observation. Thus, in the field of observation, the quality of the liquid metal can be considered controlled, in the sense that the metal can be considered free of inclusions, or sufficiently depleted in inclusions.

[0091] One objective of the invention is to take advantage of the filter effect provided by the standing acoustic wave, so as to carry out a calibration of the probe, on a portion of the casting considered to be clean, or at least of controlled inclusion quality.

[0092] The sonotrode, whether it is the same as the measuring transmitter or is different from it, is configured to emit the standing acoustic wave during an activation period Ta. The activation period Ta can last a few seconds or a few minutes.

[0093] According to one possibility, the activation period is followed by a calibration period Te, during which the standing acoustic wave is preferably not emitted, so that it is not detected by the probe receiver. The calibration period Te is sufficiently close to the activation period Ta so that it can be considered that during the calibration period, the effect of the standing acoustic wave on the liquid metal remains. Thus, during the calibration period, the observation field of the probe is considered to be free of inclusions, or sufficiently impoverished in inclusions.

[0094] It is understood that it is preferable that the calibration period be as close as possible to the activation period, so as to take advantage of the arrangement of the inclusions, outside the field of observation of the probe, under the effect of the standing acoustic wave emitted during the activation period.

[0095] Alternatively, the frequency of the standing acoustic wave is sufficiently different from the emission frequency of the probe transmitter 11, so that the influence of the standing acoustic wave can be considered negligible on the acoustic wave detected by the probe receiver.

[0096] Whatever the embodiment, during calibration, the device to be calibrated is immersed in the liquid metal and a measurement is taken at one or more calibration times t c . Each calibration instant corresponds to a calibration amplitude A t c ), which corresponds to an amplitude of the signal detected by the probe at the calibration instant t c . From the calibration amplitude or amplitudes A t c ), we define a normalization amplitude, noted A N , which corresponds to the mean or median of the calibration amplitudes (t c ).

[0097] The duration of the calibration period Ta depends on the experimental conditions. It can last from a few ms to a few hundred ms, or even a few seconds.

[0098] After the normalization amplitude A N was determined, each amplitude (t m ) resulting from the probe, at a measurement time t m , can be normalized, by a normalization function f, such that: Or :

[0099] - f corresponds to the normalization function.

[0100] - A' t m ) is the normalized amplitude

[0101] - N can be for example 20. In this case, the normalized amplitude f(A(t m ) is expressed in %. The value of 20% corresponds to a casting considered clean.

[0102] Once the normalization function has been defined for the probe, it can be used in the same way as a reference probe, whose response is known. An advantage of the method is that the value of the normalized amplitude Æ(t m ) corresponding to the normalization value N, for example 20%, represents a clean flow, regardless of the probe used. Normalization makes it possible to overcome the variability affecting the probes: the normalized amplitudes Æ(t m ) of the different probes are comparable with each other, because the normalization amplitude A N is established, for each probe, under comparable conditions, in this case in the presence of a liquid metal considered to be clean.

[0103] Standardization assumes that it is accepted that the liquid metal, considered clean, that is to say free of inclusions, or sufficiently depleted, generates acoustic waves whose amplitude is assimilated to noise. This amplitude corresponds to the standardization amplitude A N .

[0104] Normalization makes it possible to use a correlation curve as shown in Figure 4. The correlation curve is established experimentally with a reference probe, after applying the normalization function determined for said reference probe. Thus, the y-axis corresponds to f(A c ), where A ccorresponds to measured amplitudes correlated with known inclusion dimensions. The x-axis corresponds to an inclusion size, for example the diameter, measured by the reference method. The curve in Figure 4 is a response function established for the reference probe: it establishes a correlation between the standardized measurements, resulting from the reference probe, and known inclusion dimensions, established by a reference method, for example the LIMCA method.

[0105] Using the normalization function allows the response of each probe to be adjusted so that the response of each probe is comparable to the response of the reference probe.

[0106] The main steps of the invention are now described, shown diagrammatically in Figure 5.

[0107] 100 generation of a standing acoustic wave during an activation period.

[0108] During this step, a sonotrode S is used to generate the standing acoustic wave W, propagating through the liquid metal.

[0109] The sonotrode S may be different from the emitter 11, in which case it is arranged downstream or upstream of the latter. According to one possibility, the sonotrode S is coincident with the emitter 11. When the frequency of the standing acoustic wave is sufficiently different from the frequency of the acoustic wave emitted by the emitter of the probe, the activation period Ta may be coincident with all or part of the calibration period Te.

[0110] When the standing acoustic wave is likely to be confused with an acoustic wave reflected by the liquid metal, in response to an acoustic wave emitted by the probe transmitter, the calibration period Te is consecutive to the activation period Ta. The time interval between the calibration period and the activation period is then as small as possible.

[0111] Step 110: Determination of the normalization amplitude during the calibration period. During this step, a probe, as described in connection with Figures 1A and 1B, is arranged downstream of the sonotrode S, so as to carry out at least one measurement of one or more calibration amplitudes A t c ), in one or more calibration instants t c . From the calibration amplitude (t c ) or an average (or a median, or other statistical quantity) of the measurements A t c), we determine the normalization amplitude A N .

[0112] Step 120: Determination of the normalization function

[0113] From the normalization amplitude A N , the normalization function f of the probe is defined according to (1).

[0114] Steps 110 and 120 are implemented by the processing unit 32.

[0115] Step 130: Taking measurements

[0116] After the probe's normalization function has been set, the probe can be used to perform measurements at different measurement times t m . Each measurement instant corresponds to a measurement amplitude (t m ), as described in connection with Figure 2.

[0117] Step 140: Normalization

[0118] In this step, the measurement amplitude is normalized according to (1) using the normalization function resulting from step 120.

[0119] Steps 100 to 120 are preferably implemented for probes of identical design. This makes it possible to obtain a normalization function, associated with each probe. The normalized measurements of each probe are then considered comparable, the variability of each probe being corrected by the normalization function defined for each of them.

[0120] A correlation curve can be established by comparing measurements from a reference method (e.g. LIMCA) with the normalized amplitudes resulting from a probe considered as a reference probe. Such a curve can then be used for the normalized amplitudes of all probes of similar design to the reference probe.

[0121] Experimental tests

[0122] The inventors tested the trapping of inclusions by acoustophoresis in a chute. They placed a sonotrode in the center of a chute: see Figure 6A. The emission power was 100 W. Relative to Figure 6A, the dimensions are, in mm, PI = 182, P2 = 166, L1 = 55,

[0123] L2 = 146.

[0124] Downstream of the probe, a sample of the casting was taken, at a distance of 70 cm from the sonotrode. The sample was subject to a LIMCA type measurement. Figure 6B represents a concentration of inclusions (y-axis - number of inclusions, unit thousands per Kg), as a function of time (x-axis: minutes), for different inclusion sizes (diameters 20 pm; 40 pm; 60 pm; 80 pm). The sonotrode S was activated between t = 40 min and 50 min. It is observed that the concentration of detected particles decreases significantly during the activation of the sonotrode. The concentration of inclusions measured by LIMCA is representative of the inclusion quality of the casting downstream of the sonotrode. The activation of the sonotrode is accompanied by an improvement in the inclusion quality, and this for all inclusion sizes. It appears that small inclusions (20 pm) are less sensitive to acoustophoresis.

[0125] During a second test, the sonotrode was activated several times. The concentration of 20 pm inclusions downstream of the sonotrode was measured by LIMCA. Figure 6C represents a concentration of 20 pm inclusions (y-axis - number of inclusions, unit thousands per kg), for different measurement points distributed in chronological order. The sonotrode was activated between measurement points 43 - 47, 53 - 57. Between measurement points 43 and 47, the power of the sonotrode was increased to 40% of the maximum power. Between measurement points 53 and 57, the power of the sonotrode was increased to 60% of the maximum power. It is also observed that the actuation of the sonotrode, for different power levels, makes it possible to reduce the concentration of inclusions downstream of the sonotrode.

[0126] Figures 6D and 6E show the concentrations (y-axis - per thousand inclusions per kg of metal) of inclusions classified by size (x-axis - unit pm), respectively with (histogram a - black) and without (histogram b - gray) application of a transverse acoustic wave. In Figures 6D and 6E, the power of the transverse acoustic wave was 40% and 60% of the maximum power. It is observed that the effect of the transverse acoustic wave is all the more important as the size of the inclusions is large, which is consistent with the observations related to Figure 6B.

[0127] During a third test, LIMCA measurements of inclusion quality were carried out as a function of time for two castings, at the furnace outlet: during the first casting, no sonotrode was used. During a second casting, a sonotrode was used, immersed in the casting, upstream (approximately 1 m) of the area sampled for the LIMCA measurements. Figure 6F represents the concentrations of inclusions larger than 20 pm measured (y-axis - thousands of inclusions per kg) as a function of time, for each casting. For both castings, a progressive decrease in the concentration of inclusions was observed as a function of time. It was observed that whatever the time of measurement, the application of ultrasound through the casting allowed a significant reduction in the concentration of inclusions.

[0128] In Figure 6F, the points correspond to the measurements taken. A polynomial interpolation of the measurements was carried out for each of these castings, which makes it possible to obtain a continuous curve for each casting: curve a: without ultrasound - curve b: with ultrasound.

[0129] The case shown in Figure 6F corresponds to an improvement in the inclusion quality of the metal as a function of time. The acoustophoresis effect is particularly noticeable at the beginning of casting, when the metal is rich in inclusions.

[0130] In a series of tests, samples of liquid aluminum were taken during a standard casting, i.e. without the application of ultrasound, and during a test casting, downstream of a sonotrode as previously described. The samples were analyzed by a PoDFA analyzer (Porous Disk Filtration Analysis). Figure 7A and Figure 7B show metallographic images resulting from the PoDFA analysis for the standard casting and the test casting, respectively. Comparison between the two figures shows that the application of ultrasound significantly improves the inclusion state of the aluminum: there are significantly fewer oxide films on the test casting.

[0131] The experimental tests described in connection with Figures 6A to 6F and 7A to 7B confirm that the application of an ultrasonic wave makes it possible to obtain a casting in which the concentration of inclusions is limited. The invention is based on the hypothesis that whatever the casting, downstream of a standing acoustic wave, propagating transversely to the flow of the casting, the inclusion quality of a casting is comparable. Thus, downstream of the acoustophoresis acoustic wave, the amplitude of an ultrasonic wave reflected or diffracted by the metal makes it possible to carry out an amplitude adjustment, as described in connection with expression (1).

[0132] During a series of tests, a probe as described in connection with Figure 1B was implemented. The emitter 11 was used as a sonotrode producing a standing acoustic wave. Figure 8A shows a change, as a function of time, in the amplitude of waves detected by the receiver 21 of the probe. During an activation period Ta, the standing acoustic wave was emitted at a high power, in order to increase the wettability of each waveguide with respect to ultrasound. This also made it possible to impoverish the observation field of the probe by moving the inclusions towards the free surface and / or towards the walls of the chute. During the activation period, the receiver was not used, which explains why the detected amplitude is zero during the activation period.

[0133] Following the activation period, the transmitter 11 emitted an incident acoustic wave at an emission power compatible with the use of the receiver 21 of the probe. Figure 8B shows a detail of Figure 8A. It can be seen that following the activation period Ta, the measured amplitude is relatively low, due to the depletion of the observation field in inclusions resulting from the activation period. The first moments following the activation period form a calibration period Te, during which the amplitude of the pulses measured by the receiver is considered to be representative of a clean metal. In this example, the calibration period lasts 100 ms.

[0134] The invention can be applied by implementing ultrasonic measuring devices of the same type, i.e. having the same structure, the variability being due to the differences affecting the components of each device. Standardization makes it possible to limit the variability of the response between ultrasonic measuring devices of the same type.

Claims

Claims 1. Method for calibrating an ultrasonic measuring device (1), the device being intended to characterize a liquid metal, the device comprising: - a measuring transmitter (11) configured to emit an incident ultrasonic wave into the liquid metal; - a receiver (21) configured to detect an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal, following the emission of the incident ultrasonic wave; - a processing unit (32), configured to determine an amplitude of the detected ultrasonic wave; the method being characterized in that it comprises the following steps: a) arranging a sonotrode (S) in a liquid metal (2), and arranging the device (1), such that the measuring transmitter and the receiver are arranged in the liquid metal or facing the liquid metal; b) activating the sonotrode, during an activation period (Ta), so as to generate a standing acoustic wave propagating through the liquid metal; c) powering the measuring transmitter (11), such that the measuring transmitter emits an incident ultrasonic wave propagating in the liquid metal at different calibration times (t c), the calibration instants forming a calibration period (Tc); d) at each calibration instant, detection, by the receiver, of an ultrasonic wave reflected or diffracted by the liquid metal under the effect of the incident wave, and determination of the amplitude ( (t c )) of the ultrasonic wave detected by the receiver; e) calculation of a normalization amplitude (A N ) as a function of the amplitude measured at each calibration instant. f) calculation of a normalization function ( / ), configured to normalize each amplitude of an ultrasonic wave detected by the receiver by the normalization amplitude.

2. The method of claim 1, wherein the standing acoustic wave is an ultrasonic wave.

3. Method according to any one of the preceding claims, in which the device is arranged at a distance from the sonotrode of less than 2 meters.

4. A method according to any preceding claim, wherein the frequency of the standing acoustic wave is different from the frequency of the incident wave.

5. Method according to any one of the preceding claims, in which the calibration period is subsequent to the activation period.

6. Method according to claim 5 in which the sonotrode forms the measuring transmitter, the method being such that: - during step b), the measuring transmitter emits the standing acoustic wave; - during step c), the measuring transmitter and the receiver of the device are activated, so as to determine the amplitude ( (t c )) of the ultrasonic wave detected by the receiver.

7. Method according to claim 6, in which the calibration period extends for a duration less than 1s.

8. Method according to any one of the preceding claims, in which the frequency of the standing acoustic wave, generated during step b), is higher than the frequency of the incident ultrasonic wave emitted during step c).

9. Method according to any one of claims 1 to 4, in which: - the frequency of the standing acoustic wave, generated during step b), is different from the frequency of the incident ultrasonic wave emitted during step c). - the calibration period includes all or part of the activation period, all or part of steps b) and c) being implemented simultaneously.

10. Method for characterizing a liquid metal, the method using an ultrasonic measuring device (1), comprising: - a measuring transmitter (11) configured to emit an incident ultrasonic wave into the liquid metal; - a receiver (21) configured to detect an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal, following the emission of the incident ultrasonic wave; - a processing unit, configured to determine an amplitude of the detected ultrasonic wave; the method comprising the following steps: - (i) arrangement of the device (1), such that the measuring transmitter and the receiver are arranged in the liquid metal or facing the liquid metal; - (ii) powering the measuring transmitter, so that the measuring transmitter emits an incident ultrasonic wave propagating in the liquid metal; - (iii) detection, by the receiver, of an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal under the effect of the incident wave; - (iv) at a measurement instant, measurement, by the receiver, of an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal under the effect of the incident ultrasonic wave, and determination of the amplitude ( (t m )) of the ultrasonic wave detected by the receiver at each measurement instant; the method being characterized in that it comprises - (v) applying a normalization function to the amplitude determined at the measurement time, the normalization function being defined by a method according to any one of claims 1 to 9.

11. The method of claim 10, wherein - the liquid metal flows along a chute; - the transmitter and receiver are positioned facing the chute.

12. A method according to any one of claims 10 or 11, wherein - the measuring transmitter is coupled to a first waveguide (13), immersed in the liquid metal; - the receiver is coupled to a second waveguide (23), immersed in the liquid metal.

13. Ultrasonic measuring device, comprising: - a measuring transmitter (11) configured to emit an incident ultrasonic wave into the liquid metal; - a receiver (21) configured to detect an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal, following the emission of the incident ultrasonic wave; - a processing unit (32), configured to implement steps e) and f) of a method according to any one of claims 1 to 9.

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